To form an encapsulated integrated circuit (IC), a plurality of semiconductor die are conventionally arranged on a lead frame. The lead frame functions as a carrier for the plurality of die and forms the leads for each IC device. Prior to encapsulation, bond wires are attached to bond pads on the die and to the lead fingers of the lead frame. The plurality of die and the bond wires are then encapsulated to form a housing from which only the ends of the leads extend. The encapsulation protects the die and makes it possible for the ICs to be used industrially.
The die is a very sensitive component and the encapsulation must fulfill various quality requirements. For example, the encapsulation must provide the following functions:
1. Protection against physical influences (bending, shock, vibration, etc.) and support for the die. PA1 2. Sufficient electrical insulation even after extended exposure to moisture and heat. PA1 3. Resistance to the corrosive action of chemicals to which the package may be exposed. PA1 4. Sufficiently high adhesion to other surfaces. High adhesion to surfaces reduces the possibility of the lead frame separating from the encapsulation which would allow moisture to enter the package along the leads. PA1 5. High thermal conductivity so heat generated by the die will conduct to the outside of the package and dissipate. PA1 6. Sufficient stability to avoid the production of byproducts which might damage the die encased within the encapsulation. PA1 7. Resistance to variations in temperature, for example between -65.degree. C. and +200.degree. C.
To help fulfill these requirements, the production of bubbles within the encapsulation is to be avoided. One method to avoid the production of bubbles is to encapsulate the die under an appropriately high pressure. However, overly high pressures are to be avoided as a die can be damaged by excessive pressures. In particular, the bond wires, often having a diameter of less than 2.5 micrometers (.mu.m), can drift or come loose from the bond pad or lead finger during encapsulation of the die in plastic. During drifting the conductor wires are deformed by the flowing plastic, and they can touch each other or come so close to each other that the insulation resistance between them is too small and short-circuiting can occur.
Transfer molding is often used to encapsulate semiconductor die. Using the transfer molding method, lead frames with die thereon are placed into a mold with correspondingly adapted hollow spaces. The encapsulation material, such as thermoset in the form of pellets filled with fillers such as glass fibers or mineral flakes, is placed into a piston-cylinder unit, heated, and then pressed by a piston via a system of casting conduits into the hollow spaces of the form.
Transfer molding includes manufacturing using large tools with only one transfer cylinder (multi-cavity system) and on small tools with several small transfer cylinders (multi-piston system). The manufacturing with large tools has the disadvantage that the larger they are the more difficult it becomes to achieve a uniform filling of the hollow spaces of the mold. The pressure produced by the equipment must be sufficient to fill the cavity which is the most difficult to fill. This results in a high pressure at the cavities closest to the transfer cylinder which can damage the die in those cavities. Moreover, the molds become increasingly imprecise as they become larger such that it becomes difficult to achieve the desired dimensional accuracy. Since the demand on the reliability and the dimensional accuracy becomes greater with increasing device density and increasing lead density, the trend to make the tools larger to increase efficiency stands in contrast to the increased demands for accuracy. In addition, the large tools have increased down-time because of increased loading and unloading, and the additional time required for cleaning the large-surface tools.
Encapsulating devices on small tools with several small transfer cylinders makes possible shorter cycle times, improved dimensional accuracy and, on account of the shorter injection paths, a generally improved encapsulation. However, the design of an injection unit operating with several transfer cylinders is expensive as a complete injection unit (which consists of the cylinder, the piston with hydraulic mechanism and control as well as the automatic charging of the cylinder with the molding material in pellet form) is required for each two to four cavities in the mold.
The construction of the tools, the arrangement of the numerous injection units, and the supply of material are determined by the component to be produced and by the type lead frame. The tools used are therefore determined by the length of the lead frame, number of die on each lead frame, the dimensions of the lead frame and of the final package, the weight and pellet size of the encapsulation material, as well as the injection rate and injection pressure. The complete block including the injection units must be replaced upon a change of product, which is expensive.
Even though the accuracy of small transfer molding tools produces quality components, the cost-performance ratio for small tools is unfavorable for the manufacturing production quantities of semiconductor devices.
There is a need for the ability to produce high density devices which require high accuracy due to increased lead density with less complexity and increased throughput.